Under the Affordable Care Act, companies are subject to an excise tax on high-cost health plans, starting in 2018, also known as the “Cadillac tax.” Employers will have to pay a levy of 40% a year on the amount by which the cost of employee plans exceed government thresholds, which are $10,200 for individuals and $27,500 for families in the first year.

Those thresholds are low enough today that 19% of employers would have to pay the tax on at least one health plan they offer if the tax was in effect for 2015, the foundation said.

Assuming the cost of health premiums grows by 5% a year, the foundation said 26% of employers could face the tax in 2018. By 2023, that figure would rise to 30% and by 2028 it would affect 42% of employers.

Smaller firms with fewer than 200 workers were less likely to hit that threshold, the foundation said. While 25% of small firms would be affected in 2018, for example, 46% of large companies with more than 200 workers would have to pay the tax in the first year.

The study counted firms that would have at least one health plan affected by the tax. The law has special exemptions for firms with older workers and people in high-risk occupations.

The tax, meant to help fund insurance for previously uncovered Americans, is expected to generate $5 billion in revenue in 2018, and $34 billion by 2024 as more plans surpass those thresholds, according to the Congressional Budget Office.

The U.S. Treasury Department and IRS are making a "continuing effort" to get input from companies to inform guidance and final regulations on the tax, a Treasury spokeswoman said.

In order to avoid the tax in the future, companies are taking steps to reduce costs in their plans today, the foundation said. The most common reactions are increasing employee deductibles and cost sharing, eliminating covered services, capping flexible spending accounts, eliminating higher-cost health insurance options, using less expensive provider networks, and offering benefits through a private exchange, the foundation said.